This application claims the priority of German patent document DE 198 55 310.2, filed Dec. 1, 1998, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to an active suspension system for vehicles, particularly motor vehicles, having supporting assemblies which are arranged between the vehicle body and the vehicle wheels, with each supporting assembly having an actively lift-adjustable control unit and a passive spring arranged in series thereto, and having an automatic control of the control units. A path generator for generating a signal correlated with the lift position of the supporting assembly is assigned to each supporting assembly, and a sensor arrangement for generating signals which are correlated at least with vertical body accelerations is assigned to the vehicle body. Control signals used for controlling the control units have a signal fraction generated by the signals of the above-mentioned sensor arrangement.
German Patent Document DE 40 26 790 C1 illustrates a computer-controlled suspension system for motor vehicles. In this case, a control intervention, which is a function of the frequency of the suspension lifts, takes place such that, at the frequency which is characteristic of a vehicle body resonance, an increased damping of vehicle body motion will occur.
From German Patent Document DE 39 32 476 A1, a suspension system is known which has hydropneumatic spring and supporting assemblies and in which a passive spring is formed by a pneumatic spring load device. This spring load device is hydraulically connected with a piston working space of a piston-cylinder unit arranged between the sprung and the unsprung masses, in which case the "hydraulic linkage" formed by the hydraulic connection can be lengthened or shortened by feeding and removing hydraulic medium. The control signals for the control units comprise, among others, fractions which are determined by the longitudinal and lateral acceleration of the vehicle body as well as additional fractions which depend on the lifting path of the unsprung mass relative to the sprung mass as well as on the vertical acceleration of the sprung mass.
According to European Patent Document EP 0 395 114 A1, in a hydropneumatic suspension system, the suspension assemblies are controlled by a roll and pitch compensation, in which case pitch and roll motions of the vehicle body, or forces leading to corresponding motions of the body, are detected by vehicle-body-side sensors for longitudinal and lateral acceleration.
German Patent Documents DE 42 31 641 A1 and DE 44 14 022 A1 show active suspension systems, in each of which a passive mechanical spring is arranged in series with a hydraulic control assembly. According to each of these systems, the "hydraulic linkage" is combined with a mechanical spring.
According to German Patent Document DE 196 15 737 A1, in an active suspension system of a motor vehicle, vehicle body accelerations are detected in order to determine therefrom a measurement for deformation of the tires. In this manner, a comparatively exact measurement of the clearance of the vehicle body from the ground can be determined; on the one hand, the distances between the vehicle body and the vehicle wheels and, on the other hand, the conditions of the tire springs are taken into account. As a result, a realistic value of the clearance from the ground can be used as the command variable when controlling the spring system.
In addition, it is known, for example, from German Patent Document DE 43 03 160 A1 to arrange, in vehicles, between the vehicle body and the wheel, in each case only an active force generator without any passive spring arranged in series thereto, and to carry out the control as a function of parameters to be monitored by a desired-value actual-value comparison. In systems of this type, the reactivity of the force generator as well as of the control systems must be extremely high. Otherwise, an uncomfortable suspension behavior would be obtained.
It is an object of the invention to provide an active suspension system in which a special reaction to vehicle body accelerations takes place.
According to the invention, this object is achieved in that a signal fraction produced by the sensor arrangement for the vehicle body accelerations causes, for higher-frequency fractions of vertical vehicle body accelerations above a limit frequency, an adjustment of the control units reducing the respective vehicle body acceleration and, for lower frequency fractions of the vehicle body acceleration below the same or another limit frequency, an adjustment of the control units which, with respect to its amount, is reduced or imperceptible, or is opposite.
The invention is based on the general idea of processing vehicle body accelerations while taking into account their frequency spectrum. In this case, the fact is taken into account that a considerable portion of higher-frequency fractions of the vehicle body accelerations are caused by road surface unevennesses with short wavelengths, while very low-frequency fractions of the vehicle body accelerations are caused predominantly by road surface contours with very large wavelengths or by curves in the driving route, lateral gradients of the driving route in the areas of curves, and the like. As a result of the frequency-dependent consideration of the vehicle body accelerations, on the one hand, a good comfort can be achieved in that, at higher-frequency vehicle body motions, a comfort-promoting flexibility of the suspension can be ensured. On the other hand, the invention ensures secure handling of the vehicle in that vehicle body motions caused by long-wave road contours are not diminished or are diminished in a reduced manner, so that the vehicle body can maintain a defined desired position relative to the average road contour in a comparatively stable fashion.
In addition, according to a particularly preferred embodiment of the invention, while taking into account the vehicle body accelerations, for the control signals of the control assemblies, a signal fraction is also determined which is correlated with the condition of the tire springs of the vehicle wheels. If, during a drive, the payload of a vehicle remains immobile relative to the vehicle body and the inert mass of the vehicle wheels as well as of the wheel suspensions is low in comparison to the mass of the vehicle body, then the vehicle body accelerations are predominantly determined only by forces effective between the driving route and the vehicle body which are transmitted by way of the wheels and thus the tire springs and correspondingly, at least to a large extent, also determine the condition of the tire springs.
With respect to preferred characteristics of the invention, reference is made to the claims as well as to the following explanation of the drawings by which expedient embodiments of the invention are described.